Combustors are widely used in commercial operations. For example, a typical gas turbine includes at least one combustor that injects fuel into the flow of a compressed working fluid and ignites the mixture to produce combustion gases having a high temperature and pressure. The combustion gases exit the combustor and flow to a turbine where they expand to produce work.
FIG. 1 provides a simplified cross-section of a combustor 10 known in the art. A casing 12 surrounds the combustor 10 to contain the compressed working fluid. Nozzles are arranged in an end cover 16, for example, with primary nozzles 18 radially arranged around a secondary nozzle 20, as shown in FIG. 1. A liner 22 downstream of the nozzles 18, 20 defines an upstream chamber 24 and a downstream chamber 26 separated by a throat 28. The compressed working fluid flows between the casing 12 and the liner 22 to the nozzles 18, 20. The nozzles 18, 20 mix fuel with the compressed working fluid, and the mixture flows from the nozzles 18, 20 into the upstream 24 and downstream 26 chambers where combustion occurs.
During full speed base load operations, the flow rate of the fuel and compressed working fluid mixture through the nozzles 18, 20 is sufficiently high so that combustion occurs only in the downstream chamber 26. During reduced power operations, however, the primary nozzles 18 operate in a diffusion mode in which the flow rate of the fuel and compressed working fluid mixture from the primary nozzles 18 is reduced so that combustion of the fuel and the compressed working fluid mixture from the primary nozzles 18 occurs in the upstream chamber 24. During all operations, the secondary nozzle 20 operates as a combined diffusion and premix nozzle that provides the flame source for the operation of the combustor. In this manner, fuel flow through the primary and secondary nozzles 18, 20 can be adjusted, depending on the operational load of the combustor, to optimize nitrous oxide NOx emissions throughout the entire operating range of the combustor.
Various efforts have been made to design and manufacture fuel nozzles with improved premixing and diffusion capabilities, especially for higher reactivity fuels. For example, direct metal laser sintering, braising, and casting are manufacturing techniques previously used to fabricate fuel nozzles that premix the fuel and compressed working fluid prior to combustion. However, these manufacturing techniques are relatively expensive, time-consuming, and otherwise less than optimum for large-scale production. Therefore, an improved fuel nozzle that can pre-mix the fuel and compressed working fluid prior to combustion would be desirable. In addition, an improved method for making such a nozzle that utilizes less expensive machining techniques rather than other more costly techniques would be desirable.